Successful processing of polyaniline emeraldine base (PANI-EB) into useful high strength and high conductivity fibers requires solutions that are suitable for continuous fiber production. In “Conductive Polymer Compositions” by Phillip Norman Adams et al., International Publication Number: WO 99/24991, published on 20 May 1999, a fluid conductive mixture for use in the preparation of coatings, films and fibers based on polyaniline in base form doped with a sulfonic acid having in addition to at least one sulfonic acid group a second hydrogen-bonding functional group dispersed in an acid solvent having a pKa less than 4.5 but substantially higher than that of the sulfonic acid, is described. Specific examples of 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA) as the sulfonic acid and dichloroacetic acid (DCAA) spun into a competitive solvent in which the DCAA is soluble, but in which polyaniline is not soluble are disclosed. The ratio of the number of AMPSA molecules to the number of nitrogen atoms in the polyaniline as a reference was between 0.3 and 1.0; typically this ratio was 0.6. Dry polyaniline powder (Mw˜150,000 g·mol−1) was ground with AMPSA and added over a 5 min. period to the DCM under flowing nitrogen in a glove box to form a mixture having 9 mass % solids. The solution pressurized with nitrogen was extruded into the coagulant at 50±5° C. Adams et al. stated that the maximum solids content at which level gelation is not experienced is 5 mass %.
In “Inherently Electrically Conductive Fibers Wet Spun from a Sulfonic Acid-Doped Polyaniline Solution” by Stephen J. Pomfret et al., Adv. Mat. 10, 1351 (1998), it is stated that: “The primary alternative method of producing conductive polyaniline involves processing from the electrically insulating emeraldine base (EB) form, then post-doping with an aqueous protonic acid. There are disadvantages of this method: in most cases the resulting material is doped inhomogeneously; it is dedoped relatively easily; and the materials properties are usually adversely affected on doping. Processing from an inherently conductive solution, however, results in homogeneous doping, and the bulky sulfonic acids cannot be easily removed from the material afterwards.”
It is known that the performance of conducting-polymer-based devices is dependent on the properties of the dopant anion. As an example, when polyaniline fibers are used in electrochemical devices with non-aqueous electrolytes (e.g. organic solvent or ionic liquid), the sulfonic acid that is used to solublize the doped form of the polymer to enable fiber production (for example, AMPSA) inhibits the performance of the device. As stated in “Long-Lived Conjugated Polymer Electrochemical Devices Incorporating Ionic Liquids,” International Publication Number WO 02/063073 A1, as-spun AMPSA-doped polyaniline (PANI.AMPSA) fiber is a poor choice for the active electrode in electrochemical devices containing an organic solvent electrolyte (for example, LiPF6 dissolved in propylene carbonate); that is, weak electroactivity of such fibers may be deduced from the low observed currents in cyclic voltammograms thereof. Moreover, little actuation (change of length) has been observed for such fibers when a voltage is applied thereto. This is related to the large size of the AMPSA anion and, consequently, a low diffusion coefficient, thereby rendering the anions unable to exchange with the PF6− anions in the electrolyte.
Furthermore, for certain membrane-based separation applications, specific dopant anions lead to enhanced performance of polyaniline membranes. See, for example, U.S. Pat. No. 5,358,556 for “Membranes Having Selective Permeability” which issued to Kaner et al. on Oct. 25, 1994.
Accordingly, it is an object of the present invention to prepare stable, high solids content spinning solutions from high molecular weight polyaniline.
Another object of the invention is to provide a method for spinning polyaniline fiber.
Yet another object of the present invention is to provide a method for partially or totally replacing dopants present in as-spun polyaniline fibers with selected dopants in order to achieve desired characteristics of the fibers.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.